Dielectric fluid for the machining of metal by means of spark erosion

ABSTRACT

A dielectric fluid for the machining of metal by spark erosion which contains saturated aliphatic hydrocarbons with a proportion of aromatic hydrocarbons which is lower than one percent in weight and to which an additive is added, wherein organic substances from the group of anthraquinone derivatives are added as the additive in a concentration of maximally one percent in weight.

This application is a continuation of application Ser. No. 08/167,078,filed on Dec. 15, 1993 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a dielectric liquid for the machining of metalby means of spark erosion.

In the spark erosion process, an eroding machining of a workpiece takesplace by spark discharges between a working electrode and the workpiecewhich acts as the other electrode. Between the electrodes, thedielectric fluid is situated which insulates the electrodes from oneanother in such a manner that the electric breakdown does not take placebefore the smallest possible distances are obtained. In addition, thedielectric must permit the buildup of a spark as fast as possible; itmust constrict, if possible, the spark discharge channel during thedischarge for achieving a high energy density; and it must rapidlydeionize the discharge channel after the termination of the spark. Inaddition, it is used for the cooling of the working electrode and of theworkpiece as well as for the carrying-away of the contaminants which areremoved from the workpiece and, as a result of wear, from the workingelectrode.

The dielectric therefore has an important influence on the course of aspark discharge--a process which is not yet fully understood in all itsdetails and which is the subject of intensive research. The differentdemands to be made on the dielectric fluid are described, for example,in the brochure "Important Facts About Spark Erosion" of the firmOel-Held, March 1990 Edition. From this brochure, it is also known touse dielectrics on the basis of de-ionized water or of hydrocarboncompounds obtained by the distillation and refining of mineral oils orsynthetically. Known dielectric fluids for the machining of metalcontain either high concentrations of aromatic hydrocarbon compoundswhich are problematic with respect to occupational safety, or arerelatively severely limited in their erosion performance as well as,particularly during polishing erosion, with respect to the surfacequality because in this case the low energy contents of the respectiveused sparks have as a prerequisite an extremely narrow working gap whichresults in the collapse of the erosion process.

A transparent dielectric liquid of this type for spark erosion machinesis known from European Patent Document EP 0 261 546 A2. This dielectricliquid consists of a mixture of mineral oil and/or a synthetic oil and aspecific proportion of de-ionized water. In this case, paraffin oils,for example, are used as the mineral oil. Furthermore, a surface-activeadditive in a concentration of between 5 and 40 percent in weight isadded to the described mixture, which additive has multiple compounds ofcarbon atoms. When such a dielectric is used, the above-mentioneddisadvantages may occur.

German Patent Document DE 26 32 180 A1 discloses another dielectricfluid which consists, by at least 30 percent in weight, of aromaticcompounds which contain at least two homocyclic, monocyclic C₆ -ringstructures. By means of such a dielectric, the different requirementsduring the roughing-down, on the one hand, and during the planing, onthe other hand, are to be satisfactorily met in each case. However, fromthe viewpoint of occupational safety, the use of such aromatic compoundsin such concentration is problematic. The use of such a dielectrictherefore requires corresponding safety expenditures so that no toxic orallergic reactions will occur when this fluid is handled. It is known,for example, that a number of aromatic compounds belong to thecancer-causing substances. Dielectrics on a base of water or saturatedaliphatic hydrocarbons do not contain such aromatic compounds in suchhigh concentrations.

In British Patent Document GB-PS 10 03 664, the use of dielectrics isrecommended which have a carbon:hydrogen ratio of between 0.8:1 and1.8:1 and are obtained from a fractionation of coaltar. These are alsohydrocarbon mixtures with a high proportion of aromatic compounds.

U.S. Pat. No. 3,639,275 describes the addition of an additive to adielectric on a mineral oil base, the mineral oil containing between 10and 30 percent in weight of aromatic hydrocarbons and the additiveconsisting of homocyclic monocyclic hydrocarbons in the form of phenolcompounds as well as organic alkaline-earth sulfonates. This dielectrictherefore also has a high proportion of aromatic compounds. Also, theaddition of the phenol compounds which act as antioxidant agentspresents problems with respect to occupational safety.

U.S. Pat. No. 3,648,013 sees an improvement of the characteristics ofthe dielectric by the addition of an alkyl salicylic acid chromium saltas well as an alkaline earth salt of certain dialkyl sulfosuccinates ofa concentration of 1 to 10 percent in weight respectively for thepurpose of improving conductivity. The alkyl salicylic acid chromiumsalt is a monocyclic aromatic compound with a heavy metal having a toxiceffect, which today also presents problems with respect to occupationalsafety.

The adding of an additive to a dielectric on a silicone, oil orhydrocarbon base is also known from U.S. Pat. No. 3,708,422. There, theadding of aliphatic hydrocarbon amines is recommended which consist of10 to 50 carbon atoms. By means of such an additive, the metal erosionindex is to be improved, in which case the concentration of the additiveis considered to be uncritical.

It is an object of the invention to provide a dielectric fluid of theinitially mentioned type which, when it is used during spark erosion,causes a best-possible implementation of the process and particularly ahigh erosion performance while the wear of the working electrode is lowat the same time and which can be used in a comparatively unproblematicmanner with respect to occupational safety aspects.

This object is achieved by a dielectric fluid which contains saturatedaliphatic hydrocarbons with a proportion of aromatic hydrocarbons whichis lower than one percent in weight and to which an additive is added,wherein organic substances from the group of anthraquinone derivativesare added as the additive in a concentration of maximally one percent inweight. Since it consists essentially of a mixture of water andpolyhydric alcohol or of saturated aliphatic hydrocarbon compounds whichmay have a naphthenic, n-paraffinic or iso-paraffinic structure or mayconsist of a mixture of these, no high concentrations of aromaticcompounds will occur. At the same time, it was found that the additionof the additive of a higher polarizability in comparison to saturatedaliphatic hydrocarbons in a concentration of maximally one percent inweight has a very targeted positive influence on the spark erosionoperation. The addition of the additive to the dielectric base fluidtakes place not only for the improvement of the metal erosion rate orfor the reduction of the wear of the workpiece electrode. Its mainpurpose is to charge chemical satellite electrodes in the dielectric bymeans of substances which float in the dielectric fluid in a dispersedmanner and, because of the effect of the electric field between theelectrodes become stronger dipoles than the aliphatic hydrocarbons.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a two-part graph comparing the voltage and current versus timefor conventional dielectrics and dielectrics according to preferredembodiments of the invention.

FIG. 2 is a graph comparing the erosion rate versus current forconventional dielectrics and dielectrics according to preferredembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

When the voltage is applied using the dielectrics according to theinvention, these chemical satellite electrodes are aligned along thelines of electric flux of the electric field and influence the sparkdischarge in the manner illustrated in FIG. 1. This FIG. 1 illustrates,by means of a solid line, in each case the voltage course (U) and thecurrent course (I) when a conventional dielectric is used and, by meansof interrupted lines the corresponding course when a dielectricaccording to the invention is used. t_(d1) indicates the sparking delayin the case of the dielectric according to the invention, and t_(d2)indicates the sparking delay in the case of the conventional dielectric.

1.) Channels with an increased electric conducting capacity are createdin the dielectric fluid along the chemical satellite electrodes. Thedischarge bridge required for the spark breakdown is built up morerapidly, which leads to a steeper rise of the breakdown voltage (seeFIG. 1, course of the voltage U above the time) and thus to a fasterdischarge operation. This increases the erosion of material per timeunit.

2.) Also, in contrast to the previously customary spark erosion fluids,in the case of the new dielectric, the electron flow does not take placedirectly from the cathode to the anode. In this case, the electrons areattracted on their path predominantly by the dispersed chemicalsatellite electrodes and are transmitted along multiply brancheddischarge channels. Since they lose a portion of their kinetic energyduring the operation, they finally arrive on the anode with a relativelylow energy.

Because of the steep rise of the breakdown voltage, a reduction of thesparking delay is achieved at the same time (see FIG. 1). Both effectscause a reduction of up to 25% of the wear of the anode (in the case ofcoarse and medium machining stages, of the tool electrode) in comparisonto conventional dielectrics for spark erosion.

3.) During polishing erosion (finest machining stages), the workpiece isused as the anode in reversed polarity. As a result, the electrons willnow--again caused by the chemical satellite electrodes--arrive on theworkpiece with a kinetic energy which is lower in comparison toconventional dielectrics and arrive there in a comparatively widelyscattered manner. However, mainly the capability of the chemicalsatellite electrodes of accelerating the buildup of the discharge bridgebetween the electrodes results in the possibility of using a lowermedium working current for the machining of the workpiece. This isillustrated in FIG. 2 where the action of the erosion rate (V_(W)) isentered when the medium working current (I) is reduced. It is clearlydemonstrated that the course of the curve 1, which corresponds to theaction of the conventional dielectric, is situated below the curve 2which illustrates the action of the erosion rate (V_(W)) with respect tothe working current (I) of a dielectric according to the invention. Theworking current which is required for the same erosion is lower for theinvention. By means of this technique, which had not been possible bymeans of previously customary fluids, polished workpiece surfaces mayeasily be achieved which have surface roughnesses of less than 0.1micrometers. With respect to its surface quality and rapidity, thispolishing performance cannot be achieved by means of the previouslycustomary dielectrics on a base of water or saturated aliphatichydrocarbons without the addition of an additive according to theinvention.

Another advantage of the invention is the fact that it was recognizedthat the use of the highly polarizable additive in the mentioned lowconcentration positively influences also the dispersing characteristicsof the dielectric. It was observed that the dirt particles removedduring the spark discharge fly apart in the manner of an explosion outof the spark discharge range while being dispersed extremely well. Thisalso reduces the tendency to short-circuit. The reason for this gooddispersing characteristic may be that the molecules of the additivewhich are aligned in the satellite electrodes as electric dipoles causea faster distribution of the eroded metallic, that is, electricallyconductive dirt particles as a result of electric repulsive forces.

In the following, the invention will be briefly explained by means of anexample of an additive.

To a synthetically produced mixture of n- and iso-paraffins with thefollowing physical data:

    ______________________________________                                        density at 15° C.                                                                             0.7930 g/cm.sup.3                                      kinetic viscosity at 20° C.                                                                   4.3 mm.sup.2 /s                                        kinetic viscosity at 40° C.                                                                   2.8 mm.sup.2 /s                                        flash point (PM)       107° C.                                         content of aromatic compounds                                                                        <0.5% in                                                                      weight                                                 ______________________________________                                    

0.004% in weight of a dye are admixed which is called Solvent Green 5 inPart I of the known Color Index. This substance is an anthraquinonederivative. Solvent Green 5 is perylenedicarboxylic acid (9 Cl),bis-(2-methylpropyl) ester, and is also known as Fluorol Green Gold,Fluorol Green Gold 084, and Sumiplast Yellow FL 7G. It has the molecularformula C₃₀ H₂₈ O₄ and bears the CAS Registry Number 79869-59-3(indef-struc). The CAS ring system data are (01) (nr=05; sr=6, 6, 6, 6,6; ar=fr=C6.05; ir=6828-2-8) In the used amount, this substance does notpresent any problems with respect to aspects of occupational safety andwork hygiene.

During tests on a commercial spark erosion machine of the firmMAHO-Hansen GmbH, 6116 Eppertshausen, the following eroding results weremeasured:

CU-electrode 10×10 mm, power stage LS 10-3/pulse duration Ti 175-20As/pulse duty factor Tau 75-60%/eroding time=1 h.

    ______________________________________                                                         Erosion (mm.sup.3 /min)                                                                    Wear (%)                                        ______________________________________                                        Conventional Dielectric:                                                                       6.96         0.21                                            Tested Dielectric As Noted:                                                                    8.80         0.0                                             Polishing Test:  (Roughness)                                                  ______________________________________                                                                   Ra "Completely                                                      Ra after 4 h                                                                            Eroded"                                            ______________________________________                                        Conventional Dielectric:                                                                       0.19 μm                                                                              0.13 μm                                         Tested Dielectric As Noted:                                                                    0.09 μm                                                                              0.09 μm                                         ______________________________________                                    

The results illustrated in FIGS. 1 and 2 and the above-explained resultswere achieved by using a dielectric according to this example (SolventGreen).

It should be noted that the dielectric fluid according to the inventionis suitable for all filter elements which are normally used in sparkerosion systems and, because of its composition, does not have to beclassified as a flammable fluid and therefore does not representdangerous material.

Summarizing, it may be stated that, because of the addition of anadditive of high polarizability at a low concentration as a targetedcontamination of a dielectric on a base of water or on a base ofsaturated aliphatic hydrocarbons, the invention causes a significantimprovement of the spark eroding characteristics which is mainly causedby the effect of the formation of chemical satellite electrodes by thehighly polarizable molecules of the additive. While the processparameters are otherwise the same, the erosion performance risesconsiderably during the roughing; a further rise may be expected becauseof the fact that shorter pulse durations become possible between thespark discharges because of the faster rise of the breakdown voltage.While the electrode polarity is reversed, excellent polishing resultsmay be obtained within a very brief period.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A dielectric fluid for the machining of metal byspark erosion consisting essentially of saturated aliphatic hydrocarbonsand Solvent Green 5, said saturated aliphatic hydrocarbons containing nomore than 1% by weight of aromatic hydrocarbons, said Solvent Green 5being present in an amount of no more than 1% by weight, whereby themetal erosion rate is improved and wear of a work piece electrode isreduced.
 2. A dielectric fluid according to claim 1, wherein theconcentration of the Solvent Green 5 is 0.004 percent by weight.